Create prefixes for tag helpers

[NTaylorMullen]

This will involve modifying the @taghelper "assemblyname" directive to be something like @taghelper "assemblyname" "prefix".

This in turn will enforce certain restrictions on generating/understanding tag helpers.

Ex: If you do @taghelper "MyAssembly" "r-" that means that all tag helpers in _MyAssembly_ must be prefixed with _r-_.

<r-my-taghelper /> <!-- Runs the my-taghelper helper -->

Should discuss if we want to support prefixes at the C# tag helper level.

[dougbu]

nit: this looks much more like a prefix than a namespace. let's use term "prefix" going forward.

[shanselman]

I think that the attributes should be italicized in the editor so I can tell what's rendered and what isn't.

[Injac]

I prefer the standard attribute syntax. Nothing more "natural" than:

<li><a controller="Home" action="Index">Home</a></li>

This will help new developers coming from standard web-development to dive in really quick. Without prefixes.

[AnthonyPAlicea]

Personally I think 'data-asp-' or 'data-customprefix-' as a prefix feels more HTML5 standard, and less likely to be confused when integrating with client-side technologies. Great feature though!

[luisrudge]

i prefer like this:

<li><a controller="Home" action="Index">Home</a></li>

[AlexZeitler]

I would like data-asp- or asp:-prefix